Pd-catalyzed cross-coupling of α-(acyloxy)-tri-n-butylstannanes with alkenyl, aryl, and heteroaryl electrophiles

Pd-Catalyzed Arylation of Secondary α-Alkoxytricyclohexylstannanes

We have developed a general process for the formation of α-arylethers via the Pd-catalyzed arylation of secondary α-alkoxytricyclohexylstannanes. Incorporation of cyclohexyl spectator ligands into the alkylstannane and the use of the electron-deficient ligand JackiePhos (1) are critical for achieving selective alkyl transfer in this process. This system circumvents the need for a coordinating/directing oxygen-protecting group to promote selective alkyl transfer and enables α-tetrahydropyran, α-tetrahydrofuran, and open-chain secondary α-alkoxy groups to be employed efficiently in Pd-catalyzed Stille reactions with a broad range of aryl electrophiles. These findings suggest that selective transmetalation of a single marginally activated secondary alkyl unit from Sn to Pd should be broadly achievable provided that unactivated secondary alkyl ligands comprise the other three groups of the tetraalkylstannane.

Stereoselectivity in Pd-catalysed cross-coupling reactions of enantioenriched nucleophiles

Advances in Pd-catalysed cross-coupling reactions have facilitated the development of stereospecific variants enabling the use of configurationally stable, enantioenriched, main-group organometallic nucleophiles to form C(sp ³)-C(sp ²) bonds. Such stereospecific cross-coupling reactions constitute a powerful synthetic approach to attaining precise 3D control of molecular structure, allowing new stereogenic centres to be readily introduced into molecular architectures. Examples of stereospecific, Pd-catalysed cross-coupling reactions have been reported for isolable enantioenriched alkylboron, alkyltin, alkylgermanium and alkylsilicon nucleophiles. In these reactions, a single, dominant stereospecific pathway of transmetallation to palladium is required to effect efficient chirality transfer to the cross-coupled product. However, the potential for competing stereoretentive and stereoinvertive pathways of transmetallation complicates the stereochemical transfer in these processes and impedes the rational development of new stereospecific cross-coupling variants. In this Review, we describe the use of enantioenriched organometallic nucleophiles in stereospecific, Pd-catalysed cross-coupling reactions. We focus on systems involving well-defined, isolable, enantioenriched nucleophiles in which a clear stereochemical course of transmetallation is followed. Specific modes of electronic activation that influence the reactivity of alkylmetal nucleophiles are described and presented in the context of their impact on the stereochemical course of cross-coupling reactions. We expect that this Review will serve as a valuable resource to assist in deconvoluting the many considerations that potentially impact the stereochemical outcome of Pd-catalysed cross-coupling reactions.

Ligand-Free Copper(I)-Mediated Cross-Coupling Reactions of Organostannanes with Sulfur Electrophiles

The synthesis of aryl thioether through the cross-coupling of C-S bond is a highly attractive area of research due to the prevalence of aryl thioether in bioactive natural products, functional materials, agrochemicals, and pharmaceutically active compounds. Herein, we report a ligand-free Cu(I) mediated electrophilic thiolation of organostannanes with sulfur electrophiles. A selective transfer of alkyl groups was achieved in reactions with alkyl carbastannatranes affording congested thioethers. This study offers a unified method to access diaryl and aryl alkyl thioethers and was demonstrated in the context of late-stage modifications..

Catalytic and Photochemical Strategies to Stabilized Radicals Based on Anomeric Nucleophiles

Carbohydrates, one of the three primary macromolecules of living organisms, play significant roles in various biological processes such as intercellular communication, cell recognition, and immune activity. While the majority of established methods for the installation of carbohydrates through the anomeric carbon rely on nucleophilic displacement, anomeric radicals represent an attractive alternative because of their functional group compatibility and high anomeric selectivities. Herein, we demonstrate that anomeric nucleophiles such as C1 stannanes can be converted into anomeric radicals by merging Cu(I) catalysis with blue light irradiation to achieve highly stereoselective C(sp³)-S cross-coupling reactions. Mechanistic studies and DFT calculations revealed that the C-S bond-forming step occurs via the transfer of the anomeric radical directly to a sulfur electrophile bound to Cu(II) species. This pathway complements a radical chain observed for photochemical metal-free conditions where a disulfide initiator can be activated by a Lewis base additive. Both strategies utilize anomeric nucleophiles as efficient radical donors and achieve a switch from an ionic to a radical pathway. Taken together, the stability of glycosyl nucleophiles, a broad substrate scope, and high anomeric selectivities observed for the thermal and photochemical protocols make this novel C-S cross coupling a practical tool for late-stage glycodiversification of bioactive natural products and drug candidates.

Asymmetric Catalysis Using Aromatic Aldehydes as Chiral α-Alkoxyalkyl Anions

We have developed a new umpolung strategy for catalytically forming a chiral α-alkoxyalkyl anion from an aromatic aldehyde for use in asymmetric synthesis. The reaction between aromatic aldehydes and aryl or allyl electrophiles with a silylboronate utilizing a chiral copper-N-heterocyclic carbene catalyst and a palladium-bisphosphine catalyst in a synergistic manner occurred with high enantioselectivities to deliver the three-component coupling products, chiral silyl-protected secondary alcohol derivatives. Our method features the catalytic generation of enantioenriched chiral α-alkoxyalkylcopper(I) intermediates from aldehydes and their subsequent palladium-catalyzed stereospecific cross-coupling.

Rethinking Carbohydrate Synthesis: Stereoretentive Reactions of Anomeric Stannanes

In this Concept article, recent advances are highlighted in the synthesis and applications of anomeric nucleophiles, a class of carbohydrates in which the C1 carbon bears a carbon-metal bond. First, the advantages of exploiting the carboanionic reactivity of carbohydrates and the methods for the synthesis of mono- and oligosaccharide stannanes are discussed. Second, recent developments in the glycosyl cross-coupling method resulting in the transfer of anomeric configuration from C1 stannanes to C-aryl glycosides are reviewed. These highly stereoretentive processes are ideally suited for the preparation of carbohydrate-based therapeutics and were demonstrated in the synthesis of antidiabetic drugs. Next, the application of the glycosyl cross-coupling method to the preparation of Se-glycosides and to glycodiversification of small molecules and peptides are highlighted. These reactions proceed with exclusive anomeric control for a broad range of substrates and tolerate carbohydrates with free hydroxyl groups. Taken together, anomeric nucleophiles have emerged as powerful tools for the synthesis of oligosaccharides and glycoconjugates and their future applications will open new possibilities to incorporate saccharides into small molecules and biologics.

Stereospecific Stille Cross-Couplings Using Mn(II)Cl2

Cross-coupling reactions are a staple in organic synthesis, especially for C-C bond formation with sp- and sp²-carbon electrophiles. In recent years, the range of accessible C-C bonds has been extended to stereogenic centers which expedites access to greater molecular complexity. However, these reactions predominantly depend upon late transition metal (LTM) catalysts whose cost, toxicity, and/or environmental impact have come under increasing scrutiny and governmental regulation. Here, we report Mn(II)Cl₂ complexes alone, or with assistance from copper, catalyze the stereospecific cross-coupling of α-alkoxyalkylstannanes with organic electrophiles with complete retention of configuration.

Practical synthesis of pharmaceutically relevant molecules enriched in sp3 character

A highly efficient and general procedure for the preparation of medicinally relevant compounds with enhanced 3D character is reported.

Direct Synthesis of Secondary Benzylic Alcohols Enabled by Photoredox/Ni Dual-Catalyzed Cross-Coupling

An operationally simple, mild, redox-neutral method for the cross-coupling of α-hydroxyalkyltrifluoroborates is reported. Utilizing an Ir photocatalyst, α-hydroxyalkyl radicals are generated from the single-electron oxidation of the trifluoroborates, and these radicals are subsequently engaged in a nickel-catalyzed C-C bond-forming reaction with aryl halides. The process is highly selective, functional group tolerant, and step economical, which allows the direct synthesis of secondary benzylic alcohol motifs.

Glycosyl Cross-Coupling of Anomeric Nucleophiles: Scope, Mechanism, and Applications in the Synthesis of Aryl C-Glycosides

Stereoselective manipulations at the C1 anomeric position of saccharides are one of the central goals of preparative carbohydrate chemistry. Historically, the majority of reactions forming a bond with anomeric carbon has focused on reactions of nucleophiles with saccharide donors equipped with a leaving group. Here, we describe a novel approach to stereoselective synthesis of C-aryl glycosides capitalizing on the highly stereospecific reaction of anomeric nucleophiles. First, methods for the preparation of anomeric stannanes have been developed and optimized to afford both anomers of common saccharides in high anomeric selectivities. We established that oligosaccharide stannanes could be prepared from monosaccharide stannanes via O-glycosylation with Schmidt-type donors, glycal epoxides, or under dehydrative conditions with C1 alcohols. Second, we identified a general set of catalytic conditions with Pd₂(dba)₃ (2.5 mol%) and a bulky ligand (JackiePhos, 10 mol%) controlling the β-elimination pathway. We demonstrated that the glycosyl cross-coupling resulted in consistently high anomeric selectivities for both anomers with mono- and oligosaccharides, deoxysugars, saccharides with free hydroxyl groups, pyranose, and furanose substrates. The versatility of the glycosyl cross-coupling reaction was probed in the total synthesis of salmochelins (siderophores) and commercial anti-diabetic drugs (gliflozins). Combined experimental and computational studies revealed that the β-elimination pathway is suppressed for biphenyl-type ligands due to the shielding of Pd(II) by sterically demanding JackiePhos, whereas smaller ligands, which allow for the formation of a Pd-F complex, predominantly result in a glycal product. Similar steric effects account for the diminished rates of cross-couplings of 1,2-cis C1-stannanes with aryl halides. DFT calculations also revealed that the transmetalation occurs via a cyclic transition state with retention of configuration at the anomeric position. Taken together, facile access to both anomers of various glycoside nucleophiles, a broad reaction scope, and uniformly high transfer of anomeric configuration make the glycosyl cross-coupling reaction a practical tool for the synthesis of bioactive natural products, drug candidates, allowing for late-stage glycodiversification studies with small molecules and biologics.

Stereospecific Palladium-Catalyzed Acylation of Enantioenriched Alkylcarbastannatranes: A General Alternative to Asymmetric Enolate Reactions

We report the development of a Pd-catalyzed process for the cross coupling of unactivated primary, secondary, and tertiary alkylcarbastannatrane nucleophiles with acyl electrophiles. Reactions involving optically active alkylcarbastannatranes occur with exceptional stereofidelity and with net retention of absolute configuration. Because the stereochemistry of the resulting products is entirely reagent-controlled, this process may be viewed as a general, alternative approach to the preparation of products typically accessed via asymmetric enolate methodologies. Additionally, we report a new method for the preparation of optically active alkylcarbastannatranes, which should facilitate their future use in stereospecific reactions.

Efficient cross-coupling of aryl/alkenyl triflates with acyclic secondary alkylboronic acids

Efficient cross-coupling between aryl/alkenyl triflates and acyclic secondary alkylboronic acids is realized by employing P,PO ligand L1/L2, providing a series of sterically congested acyclic secondary alkyl arenes/olefins in good to excellent yields. The method has enabled a concise and 4-step synthesis of a gossypol intermediate.

A regioselective double Stille coupling reaction of bicyclic stannolanes

A regioselective double Stille coupling reaction was explored using bicyclic stannolanes that were easily prepared from the radical cascade reaction of β-amino-α-methylene esters. Various 1-bromo-2-iodoarenes underwent the double coupling reaction to afford benzoisoindole derivatives in a regioselective manner, where the carbon attached to the iodine selectively coupled with the vinylic carbon, and then the carbon attached to bromine coupled with the alkyl carbon. The combination of intra- and intermolecular coupling reactions provided hexahydroindeno[1,2-b]pyrrole derivatives in good yields. The yields were further improved in the presence of excess amounts of CsF. An attempt to identify the reaction intermediate was made wherein the decomposition of the stannolanes with aqueous HCl and HBr afforded trigonal bipyramidal (TBP) pentacoordinated tin complexes, as confirmed by microanalyses and (119)Sn NMR. Using DCl for the decomposition selectively introduced a deuterium to the E-position of the exomethylene unit. The complexes smoothly underwent the intramolecular Stille coupling reaction in the presence of both a palladium catalyst and DABCO, affording hexahydroindeno[1,2-b]pyrroles in good yields. These results suggest that the double coupling reaction progresses through a TBP tin complex, promoting the second intramolecular coupling reaction between the aryl halide and Csp(3)-tin bond.

Transition-Metal-Free Stereospecific Cross-Coupling with Alkenylboronic Acids as Nucleophiles

We herein report a transition-metal-free cross-coupling between secondary alkyl halides/mesylates and aryl/alkenylboronic acid, providing expedited access to a series of nonchiral/chiral coupling products in moderate to good yields. Stereospecific SN2-type coupling is developed for the first time with alkenylboronic acids as pure nucleophiles, offering an attractive alternative to the stereospecific transition-metal-catalyzed C(sp(2))-C(sp(3)) cross-coupling.

Single-Electron Transmetalation: Protecting-Group-Independent Synthesis of Secondary Benzylic Alcohol Derivatives via Photoredox/Nickel Dual Catalysis

Protecting-group-independent cross-coupling of α-alkoxyalkyl- and α-acyloxyalkyltrifluoroborates with aryl and heteroaryl bromides is achieved through application of photoredox/nickel dual catalysis. Reactions occur under exceptionally mild conditions, with outstanding functional group compatibility and excellent observed tolerance of heteroarenes. This method offers expedient access to protected secondary benzylic alcohol motifs bearing benzyl, pivaloyl, and N,N-diisopropylcarbamoyl protecting groups.

Configurationally Stable, Enantioenriched Organometallic Nucleophiles in Stereospecific Pd-Catalyzed Cross-Coupling Reactions: An Alternative Approach to Asymmetric Synthesis

Several research groups have recently developed methods to employ configurationally stable, enantioenriched organometallic nucleophiles in stereospecific Pd-catalyzed cross-coupling reactions. By establishing the absolute configuration of a chiral alkyltin or alkylboron nucleophile prior to its use in cross-coupling reactions, new stereogenic centers may be rapidly and reliably generated with preservation of the known initial stereochemistry. While this area of research is still in its infancy, such stereospecific cross-coupling reactions may emerge as simple, general methods to access diverse, optically active products from common enantioenriched organometallic building blocks. This minireview highlights recent progress towards the development of general, stereospecific Pd-catalyzed cross-coupling reactions using configurationally stable organometallic nucleophiles.

SmI2-mediated dimerization of indolylbutenones and synthesis of the myxobacterial natural product indiacen B

The synthesis and reactivity of indole derivatives substituted in the benzene section was studied. Starting materials 4- and 6-iodoindole were conveniently prepared via the Batcho–Leimgruber route and purified by sublimation. Novel vicinally indolyl-substituted cyclopentanols with unexpected cis-configuration were formed by SmI₂-mediated reductive dimerization of a 4-(indol-6-yl)butenone, obtained by Heck reaction. The two indolyl units appear to chelate Sm(II)/(III) leading to a gauche-type arrangement at the newly formed bond between the two β-carbons. Through a sequence of Sonogashira cross coupling and Meyer–Schuster rearrangement 6-prenoylindole was synthesized and reductively dimerized to a cyclopentane in a [3 + 2] cycloaddition by treatment with SmI₂ in THF. From 4-iodoindole, the natural product indiacen B from the myxobacterium Sandaracinus amylolyticus was synthesized for the first time, confirming its antimicrobial activity. The E-configuration of the chloroalkene moiety of indiacen B was confirmed by X-ray analysis.

Photoredox Cross-Coupling: Ir/Ni Dual Catalysis for the Synthesis of Benzylic Ethers

Single-electron transmetalation has emerged as an enabling paradigm for the cross-coupling of Csp(3) hybridized organotrifluoroborates. Cross-coupling of α-alkoxymethyltrifluoroborates with aryl and heteroaryl bromides has been demonstrated by employing dual catalysis with a combination of an iridium photoredox catalyst and a Ni cross-coupling catalyst. The resulting method enables the alkoxymethylation of diverse (hetero)arenes under mild, room-temperature conditions.

Copper-catalyzed asymmetric three-component borylstannation: enantioselective formation of C-Sn bond

In summary, a first copper-catalyzed synthesis of α-aryl-β-borylstannane compounds was accomplished through three-component borylstannation of aryl-substituted alkenes. In the exploration of an asymmetric variant, chiral sulfinylphosphine ligands proved advantageous in controlling stereochemistry of B-Cu addition and in promoting transmetalation of enantioenriched alkyl-Cu species. The stereochemical outcome supported a sequential syn-borylcupration and configuration-retentive transmetalation mechanism. Moreover, α-chiral β-borylstannanes were easily transformed into a diverse array of secondary alkylstannanes and triarylethane with high enantiomeric purity. The applications of chiral sulfinylphosphine ligands to other tandem Cu-B addition reactions are currently under investigation in our group.

Carbastannatranes: a powerful coupling mediators in Stille coupling

Carbastannatranes are better coupling mediators in Stille coupling due to easy transfer of axial Sn-bound hydrocarbyl substituent during transmetallation step.

Rh-catalyzed addition of β-carbonyl pinacol alkylboronates to aldehydes: asymmetric synthesis of γ-butyrolactones

The rhodium-catalyzed 1,2-addition of chiral benzylic secondary alkylboronic esters with a coordinating carbonyl group to aldehydes was demonstrated with high levels of enantiospecificity. Pinacol boronic ester derivatives can be employed directly for the addition in the presence of KHF2 without the use of corresponding trifluoroborate salts where retention of the configuration was observed. Enantiomerically enriched β,γ-diaryl-substituted γ-butyrolactones were synthesized in good yields.

Stereoretentive Pd-catalysed Stille cross-coupling reactions of secondary alkyl azastannatranes and aryl halides

The development of transition metal-catalysed cross-coupling reactions has greatly influenced the manner in which the synthesis of complex organic molecules is approached. A wide variety of methods are now available for the formation of C(sp(2))-C(sp(2)) bonds, and more recent work has focused on the use of C(sp(3)) electrophiles and nucleophiles. The use of secondary and tertiary alkyl nucleophiles in cross-coupling reactions remains a challenge because of the propensity of these alkyl groups to isomerize under the reaction conditions. Here, we report the development of a general Pd-catalysed process for the stereoretentive cross-coupling of secondary alkyl azastannatrane nucleophiles with aryl chlorides, bromides, iodides and triflates. Coupling partners with a wide range of electronic characteristics are well tolerated. The reaction occurs with minimal isomerization of the secondary alkyltin nucleophile, and with retention of absolute configuration. This process constitutes the first general method to use secondary alkyltin reagents in cross-coupling reactions.

Stereospecific cross-coupling of secondary organotrifluoroborates: potassium 1-(benzyloxy)alkyltrifluoroborates

Potassium 1-(alkoxy/acyloxy)alkyltrifluoroborates have been synthesized through a copper-catalyzed diboration of aldehydes and subsequent conversion of the resulting potassium 1-(hydroxy)alkyltrifluoroborates. The palladium-catalyzed Suzuki-Miyaura reaction employing the potassium 1-(benzyloxy)alkyltrifluoroborates with aryl and heteroaryl chlorides provides access to protected secondary alcohols in high yields. The β-hydride elimination pathway is avoided through use of the benzyl protecting group, which is proposed to stabilize the diorganopalladium intermediate by coordination of the arene to the metal center. This cross-coupling is stereospecific with complete retention of stereochemistry.

Inversion or retention? Effects of acidic additives on the stereochemical course in enantiospecific Suzuki-Miyaura coupling of α-(acetylamino)benzylboronic esters

The stereochemical course of the stereospecific Suzuki-Miyaura coupling of enantioenriched α-(acetylamino)benzylboronic esters with aryl bromides can be switched by the choice of acidic additives in the presence of a Pd/XPhos catalyst system. Highly enantiospecific, invertive C-C bond formation takes place with the use of phenol as an additive. In contrast, high enantiospecificity for retention of configuration is attained in the presence of Zr(Oi-Pr)(4)·i-PrOH as an additive.